Recombinant Adeno-Associated Viruses (rAAV) based on AAV serotype 2 (AAV2) have become recognized as vehicles of choice for many gene-delivery applications for several reasons. Among these, AAV2 are able to transduce and cause persistent transgene expression in several different cell types including both dividing and quiescent cells without normally causing pathology or a vigorous cell-mediated immune response (reviewed in He et al., Proceedings of the National Academy of Sciences, 95:2509–2514, 1998; Monahan et al., Gene Ther. 7:24–30, 2000; and Rabinowitz et al., Virology 278:301–308, 2000). Although the broad host cell tropism of rAAV2 virions is advantageous in many situations, it is often undesirable in others. In addition, the broad cell tropism of rAAV2 often requires an impractically large dose of virions to induce a therapeutic effect as virions infect both target and non-target cells. A major goal in this field has thus been to develop rAAV that selectively target only a particular cell type or tissue. Such cell-selective rAAV would reduce or eliminate non-target cell transduction and also thereby reduce the viral dose required to cause a therapeutic effect.
Producing cell- or tissue-specific AAV virions, however, has proven to be a difficult undertaking. A major reason for this is that until very recently (Xie et al, Proceedings of the National Academy of Sciences, 2002, v. 99, p.10405–10410), no structure had been resolved for any AAV serotype. The structural information that was available was inferred from parvovirus sequence homologies (Girod et al., Nat. Med. 5:1052–1056, 1999; and Grifman et al., Mol. Ther.3:964–975, 2001) and mutational analyses of the AAV2 capsid gene (cap). Through the latter technique, the amino acid residues of AAV2 that are critical for binding to AAV's primary cellular receptor (the heparan sulfate proteoglycan receptor) were identified. Despite this advance, methods for altering AAV tropism have not been optimized. For example, some rAAV virions made to express particular homing peptides exhibit packaging problems. Such problems underscore the need for a new, more practical approach for creating rAAV with restricted tropisms.